sFLCs may be a useful additional marker to monitor WM. Their short serum half-life and the large clinical range provide a sensitive marker for assessment of response to treatment .
Leleu et al.  studied the use of sFLCs to monitor response to treatment in 48 WM patients (untreated [n=20] or relapse and/or refractory [n=28]), participating in a trial of bortezomib and rituximab treatment. The proportion of patients who demonstrated a response to therapy was higher using sFLC analysis (79%, defined as a ≥50% decrease in iFLC from baseline) than monoclonal protein quantification by SPE (60%). Similar results were obtained when the difference between the iFLC and uninvolved (uFLC) sFLC concentrations (dFLC) was used as an alternative measure of sFLC response (κ-statistic 0.89). In addition, the time to response was shorter when assessed using iFLC compared to the monoclonal protein response (2.1 vs 3.7 months, p=0.05) . There was a significant correlation of progression as defined by iFLC or SPE criteria (a >25% increase from maximum response), with 81% of patients showing concordance of both markers (κ-statistic: 0.63). The median time to progression (TTP) was shorter as measured by iFLC than by following SPE criteria (13.7 vs. 18.9 months, respectively). The authors concluded that iFLC was a sensitive marker for the early determination of response and progression in WM.